How to Size Heating Radiators and Air Conditioning BTU Capacity
Properly sizing heating radiators and air conditioners ensures optimal indoor comfort without inflating your energy bills. An undersized unit will run continuously at full blast, while an oversized unit causes frequent cycling, poor dehumidification, and higher upfront costs.
Thermal Sizing Formula: Watts, Volume & Insulation
The thermal load formula calculates power as: Power (Watts) = Room Volume (m³) × Insulation Factor (W/m³). For air conditioning, internal heat gains from occupants and electronic appliances as well as solar exposure are factored in.
P (Watts) = Volume (m³) × Coefficient Déperdition (W/m³)Coefficient de 30 W/m³ (RE 2020) à 50 W/m³ (passoire thermique sans isolation).
BTU/h = Puissance thermique en Watts × 3,412Avec majoration de +100 W par occupant et compensation d'ensoleillement Sud/Ouest.
BTU, Watts, Frigories and Floor Area Equivalence Chart
Guide des capacités thermiques indicatives pour une hauteur sous plafond de 2,50 m et isolation standard
| Floor Area | Volume (2.50m ceiling) | Heating Capacity (W) | Cooling Capacity (BTU/h) | Cooling (Watts) | Frigories / hour |
|---|---|---|---|---|---|
| 10 m² (Chambre / Bureau) | 25 m³ | 800 - 1 000 W | 7 000 BTU/h | 2 050 W | 1 750 fg/h |
| 15 m² (Chambre parentale) | 37,5 m³ | 1 200 - 1 500 W | 9 000 BTU/h | 2 500 W | 2 200 fg/h |
| 25 m² (Salon standard) | 62,5 m³ | 2 000 - 2 500 W | 12 000 BTU/h | 3 500 W | 3 000 fg/h |
| 35 m² (Séjour ouvert) | 87,5 m³ | 2 800 - 3 500 W | 18 000 BTU/h | 5 200 W | 4 500 fg/h |
| 50 m² (Grand espace / Loft) | 125 m³ | 4 000 - 5 000 W | 24 000 BTU/h | 7 000 W | 6 000 fg/h |
Frequently Asked Questions About Heating and Cooling Sizing
What is the common '100 Watts per square meter' rule of thumb?
The 100 W/m² rule is a historic approximation for average insulated homes in temperate climates with standard 2.5m ceiling heights. Modern well-insulated homes (Passivhaus / RE 2020) only need 60 to 70 W/m², whereas poorly insulated older homes may require up to 130 W/m².
What is a BTU and how do I convert BTU to Watts?
BTU (British Thermal Unit) measures cooling capacity. 1 Watt equals approximately 3.412 BTU/h. For instance, a standard 12,000 BTU/h mini-split corresponds to about 3,500 Watts (3.5 kW) of thermal cooling.
What is a Frigorie?
A Frigorie (fg/h) is a metric unit of refrigeration widely used in Spain and Latin America representing the extraction of 1 kilocalorie of heat. 1 Frigorie/h equals approximately 1.163 Watts.
Why do attic rooms need higher air conditioning power?
Roof surfaces absorb direct intense solar radiation throughout the summer. Warm air also naturally rises to top floors. Adding a 15% to 20% power buffer is essential for attic conversions.
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